Analog-to-digital converters (A/D converters) are well known devices for providing a digital representation of an analog voltage at the A/D input. Typical A/D converters provide a 16-bit output, but this is not fundamental. However, depending upon the number of bits resolution provided by the A/D converter, the resolution is limited.
Certain classes of input signal that must be digitized are characterized by a high ratio between maximum and minimum values. An example is the signal from a Fourier transform spectrometer. This signal, which varies as a function of the position of the interferometer's moving mirror, is characterized by a relatively huge central peak (referred to as a centerburst) occurring over a narrow range of mirror position. If the A/D converter is set so that the maximum voltage (which occurs at the centerburst) provides nearly the maximum digital output, the resolution of the output will be quite low over most of the mirror's travel. Currently, the problem is addressed by using gain ranging near the centerburst. This technique entails reducing (for example, by a factor of 16) the analog voltage that is input to the A/D converter when the mirror is in the vicinity of the centerburst position. However, it then becomes impossible to measure noise in the centerburst region.